1. Field of the Invention
The invention relates in general to an amplifier device, and more particularly to an amplifier device capable of suppressing noise coupling.
2. Description of the Related Art
During operation of a conventional amplifier, external noise may often enter the output end and pass through one or more internal phase compensation circuits within the amplifier to be coupled to other internal parts of the amplifier, resulting in unexpected phenomena such as abnormal current leakage, abnormal output waveform and abnormal operation.
Referring to FIG. 1A, a circuit diagram of a conventional amplifier is shown. The conventionally amplifier is, for example, used in the sample-and-hold circuit or the source driver of a liquid crystal display. If the voltage at a loading end Out changes (that is, external noise occurs), external noise entering the loading end Out will pass through an output switch S1 to be coupled to an output end of the amplifier device 100. Then, the coupled noise enters the internal loop and passes through an internal phase compensation circuit 102 or 104 to be coupled to other internal parts of the amplifier device 100. Thus, abnormal current leakage or abnormal waveform will occur at the loading end Out.
Referring to FIG. 1B, a signal timing diagram of the amplifier circuit 100 of FIG. 1A explains how the external noise causes the aforementioned abnormal phenomena to the amplifier device 100. When the input voltage Vin of the amplifier device 100 changes to a high level from a low level, the switch S1 will be temporarily turned off (OFF), and the interior of the amplifier device 100 will follow the high level of the input signal through a close loop and become locked. Consequently, the voltage Vout at the output end is boosted, but the voltage at the loading end Out still remains at the low level because the switch S1 is already turned off.
Then, the switch S1 will be turned on (changing to the ON state from the OFF state), leading to redistribution of charges between the output end and the loading end Out. The voltage level of the output voltage Vout is then lowered down, further pulling down the voltage levels of the feedback nodes N1 and N2 of the internal phase compensation circuits 102 and 104 of the amplifier device 100 via coupling. Responsively, the transistor M14 is turned off, so that the current of the transistor M15 is boosted but the current of the transistor M16 is lowered down. As a result, the output voltage Vout is boosted, and it returns to its original level to continue to charge the loading end Out.
However, when the amplifier device 100 continues to charge the loading end Out, a second coupling (noise) occurs and results in abnormal phenomena. In greater details, when the amplifier device 100 charges the loading end Out and boosts the voltage level at the loading end Out, the voltages at the node N1 and N2 will be boosted due to the second coupling (designated by the dotted line region in the left hand side of FIG. 1B), which further causes the transistor M12 to be turned off. Thus, the current of the transistor M16 will increase, that is, extra current leakage occurs to the DC current flowing through the transistors M15 and M16. As a result, the waveform of the loading voltage Out experiences abnormal drop during the charging period, and the charging speed will slow down. As indicated in FIG. 1B, during the period when Vin is at a high level, the actual waveforms (designated in solid lines) of the output voltage Vout and the loading voltage at the loading end Out are abnormal with respect to the ideal waveform (designated in dotted lines).
Likewise, after the input voltage Vin changes to a low level from a high level and the switch S1 is turned on (changing to the ON state from the OFF state), the amplifier device 100 discharges the loading end Out, causing the voltages at the nodes N1 and node N2 (designated by the dotted line region in the right hand side of FIG. 1B) to be lowered down due to noise coupling, which further causes the transistor M10 to be turned off. Thus, the current of the transistor M15 will be boosted, that is, extra current leakage occurs to the DC current flowing through the transistors M15 and M16. As a result, the waveform of the loading voltage at the loading end Out will experience abnormal increase during the discharging period, and the discharging speed will slow down. As indicated in FIG. 1B, during the period when Vin is at a low potential, the actual waveforms (designated in solid lines) of the output voltage Vout and loading voltage at the loading end Out are abnormal with respect to the ideal waveform (designated in dotted lines).
Apart from the external noise occurring when the input voltage changes and the loading end is charged/discharged, any other type of external interference noise coming from the environment may also be coupled to the feedback node N1 or N2 of the amplifier through the output end and the phase compensation circuit 102 or 104 and result in abnormal circuit operations.
To summarize, the conventional amplifier may often experience abnormal output waveform, abnormal current leakage, and abnormal circuit operation when an external signal or noise is coupled to the phase compensation circuit and causes interference to the operation of other internal parts of the amplifier.